1. Field of the Invention
The present invention relates to a thick-film thermal printhead which allows smooth transfer of a recording paper. The present invention also relates to a method of making such a thick-film thermal printhead.
2. Description of the Related Art
As is well known, thermal printheads are generally divided into the thick-film type and the thin-film type. A thick-film type thermal printhead includes a heating resistor which is larger in thickness than that of a thin-film thermal printhead. FIGS. 15 and 16 of the accompanying drawings illustrate an example of prior art thick-film thermal printhead. The illustrated printhead includes an insulating head substrate 11, a common electrode 12, a plurality of individual electrodes 13 and a linear heating resistor 14. The heating resistor 14 has a predetermined width and extends longitudinally of the head substrate 11. As shown in FIG. 15, the common electrode 12 includes a plurality of comb-teeth 12a, and a main conductor 12b to which the comb-teeth 12a are commonly connected. The teeth 12a and the individual electrodes 13 are alternately arranged. The heating resistor 14 extends across the teeth 12a and the individual electrodes 13. The distance between one edge 14a of the heating resistor 14 and the main conductor 12b is indicated by a reference sign Sxe2x80x2.
As shown in FIG. 16, the main conductor 12b is provided with an auxiliary conductor 15. The auxiliary conductor 15, which is elongated in parallel with the heating resistor 14, prevents a voltage drop at the common electrode 12. As shown in this figure, there is provided a protective film 16 which covers the auxiliary conductor 15 as well as the common electrode 12, the individual electrodes 13 and the heating resistor 14. Due to the existence of the heating resistor 14 and the auxiliary conductor 15, the protective film 16 includes a first projection 16a and a second projection 16b projecting toward a platen roller R. In operation, the platen roller R presses a recording paper P against the first projection 16a for conducting required printing.
For smoothly moving the recording paper P along a paper transfer path, it is required that the paper P does not come into contact with the second projection 16b during the printing. Therefore, in the prior art printhead, by making the distance Sxe2x80x2 (between the heating resistor 14 and the main conductor 12b) larger (e.g. 200-300 xcexcm) than the width of the heating resistor 14, the auxiliary conductor 15 is sufficiently spaced from the heating resistor 14 for making the second projection 16b spaced from the paper transfer path as much as possible.
Although the prior art printhead takes the above-described measures, it still has the following drawbacks.
A first drawback is as follows. In printing, heat is generated at the heating resistor 14. Part of the heat is transmitted directly to the head substrate 11 for dissipation, whereas other part of the heat is transmitted to the head substrate 11 via the common electrode 12 or the individual electrodes 13 for dissipation. However, when the distance Sxe2x80x2 is increased as described above, the length of each tooth 12a of the common electrode 12 correspondingly increases. This causes difficulty in transmitting the heat generated at the heating resistor 14 to the main conductor 12b via the teeth 12a. As a result, the teeth 12a heated by the heating resistor 14 are caused to have a high temperature at portions corresponding to the distance Sxe2x80x2. This high-temperature state is shown by a curve Dxe2x80x2 in a graph of FIG. 14. This graph generally illustrates how the temperature of the teeth 12a varies in accordance with a distance from the heating resistor 14.
Due to the influence of the teeth 12a in a high-temperature state as described above, the recording paper P transferred by the platen roller R is exposed to the high temperature in moving the distance Sxe2x80x2. As is well known, the paper P often generates dust when exposed to a high temperature. When such paper dust adheres to the protective film 16 of the printhead, it hinders the paper P from smoothly sliding over the protective film 16, which may bar smooth paper transfer. Since the generation of paper dust increases in proportion to the printing speed of the printer, the printing speed need be limited to not more than 3 inch/sec in an apparatus using the prior-art printhead.
Another drawback is as follows. As described above, the recording paper P is transferred along the transfer path in press contact with the first projection 16a of the protective film 16. At this time, unpleasant noise may be made due to strong rubbing of the recording paper P with the first projection 16a. Further, friction between the recording paper P and the first projection 16a may cause the recording paper P to thermally adhere to the protective film 16 (generation of sticking), which may prevent the smooth transfer of the paper P.
The present invention has been conceived under the circumstances described above. It is therefore an object of the present invention to provide a thermal printhead which is capable of smoothly transferring a recording paper. Another object of the present invention is to provide a method of making such a thermal printhead.
A thermal printhead provided in accordance with a first aspect of the present invention comprises an insulating substrate, a common electrode formed on the substrate, a linear heating resistor electrically connected to the common electrode, and a plurality of individual electrodes electrically connected to the heating resistor. The common electrode includes a plurality of comb-teeth and a main conductor connected to the comb-teeth. The heating resistor has a predetermined width. The distance between the main conductor and the heating resistor is smaller than the width of the heating resistor.
Preferably, the distance between the main conductor and the heating resistor may be no less than 0.25 times but no more than 0.75 times the width of the heating resistor.
Preferably, the thermal printhead according to the present invention may further comprise an auxiliary conductor provided on the main conductor.
Preferably, the distance between the auxiliary conductor and the heating resistor may be no less than twice the distance between the main conductor and the heating resistor.
Preferably, the thermal printhead according to the present invention may further comprise a protective coating for covering the heating resistor.
The protective coating may have a flat exposed surface.
Preferably, the thermal printhead according to the present invention may further comprise a glaze layer which supports the heating resistor and has a crescent cross section. The protective coating may include a bulging portion following the glaze layer.
Preferably, the glaze layer and the bulging portion may have respective curved profiles which are cross-sectionally parallel with each other.
Preferably, the protective coating includes a first protective film directly contacting the heating resistor and a second protective film formed on the first protective film.
Preferably, the first protective film may include a thin-wall portion located adjacent to the heating resistor.
Preferably, the first protective film and the second protective film may be formed of a same glass material. Alternatively, the first protective film may be made of glass, whereas the second protective film may be made of sialon.
A method of making a thermal printhead provided in accordance with a second aspect of the present invention comprises a step of preparing an insulating substrate, a step of forming on the substrate a conductor pattern and a heating resistor electrically connected to the conductor pattern, a step of forming a first protective film for covering the heating resistor, a step of removing a bulging portion formed in the first protective film due to the heating resistor and a step of forming a second protective film on the first protective film.
Other features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings.